Class: MCP::Server::Transports::StreamableHTTPTransport

Inherits:
Transport
  • Object
show all
Defined in:
lib/mcp/server/transports/streamable_http_transport.rb

Constant Summary collapse

SSE_HEADERS = 
{
  "Content-Type" => "text/event-stream",
  "Cache-Control" => "no-cache",
  "Connection" => "keep-alive",
}.freeze
REQUIRED_POST_ACCEPT_TYPES = 
["application/json", "text/event-stream"].freeze
REQUIRED_GET_ACCEPT_TYPES = 
["text/event-stream"].freeze
STREAM_WRITE_ERRORS = 
[IOError, Errno::EPIPE, Errno::ECONNRESET].freeze
SESSION_REAP_INTERVAL = 
60

Instance Method Summary collapse

Methods inherited from Transport

#handle_json_request, #open, #send_response

Constructor Details

#initialize(server, stateless: false, session_idle_timeout: nil) ⇒ StreamableHTTPTransport

Returns a new instance of StreamableHTTPTransport.



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# File 'lib/mcp/server/transports/streamable_http_transport.rb', line 16

def initialize(server, stateless: false, session_idle_timeout: nil)
  super(server)
  # Maps `session_id` to `{ get_sse_stream: stream_object, server_session: ServerSession, last_active_at: float_from_monotonic_clock }`.
  @sessions = {}
  @mutex = Mutex.new

  @stateless = stateless
  @session_idle_timeout = session_idle_timeout
  @pending_responses = {}

  if @session_idle_timeout
    if @stateless
      raise ArgumentError, "session_idle_timeout is not supported in stateless mode."
    elsif @session_idle_timeout <= 0
      raise ArgumentError, "session_idle_timeout must be a positive number."
    end
  end

  start_reaper_thread if @session_idle_timeout
end

Instance Method Details

#close ⇒ Object 



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# File 'lib/mcp/server/transports/streamable_http_transport.rb', line 55

def close
  @reaper_thread&.kill
  @reaper_thread = nil

  removed_sessions = @mutex.synchronize do
    @sessions.each_key.filter_map { |session_id| cleanup_session_unsafe(session_id) }
  end

  removed_sessions.each do |session|
    close_stream_safely(session[:get_sse_stream])
    close_post_request_streams(session)
  end
end

#handle_request(request) ⇒ Object 



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# File 'lib/mcp/server/transports/streamable_http_transport.rb', line 42

def handle_request(request)
  case request.env["REQUEST_METHOD"]
  when "POST"
    handle_post(request)
  when "GET"
    handle_get(request)
  when "DELETE"
    handle_delete(request)
  else
    method_not_allowed_response
  end
end

#send_notification(method, params = nil, session_id: nil, related_request_id: nil) ⇒ Object 



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# File 'lib/mcp/server/transports/streamable_http_transport.rb', line 69

def send_notification(method, params = nil, session_id: nil, related_request_id: nil)
  # Stateless mode doesn't support notifications
  raise "Stateless mode does not support notifications" if @stateless

  notification = {
    jsonrpc: "2.0",
    method: method,
  }
  notification[:params] = params if params

  streams_to_close = []

  result = @mutex.synchronize do
    if session_id
      # Send to specific session
      if (session = @sessions[session_id])
        stream = active_stream(session, related_request_id: related_request_id)
      end
      next false unless stream

      if session_expired?(session)
        cleanup_and_collect_stream(session_id, streams_to_close)
        next false
      end

      begin
        send_to_stream(stream, notification)
        true
      rescue *STREAM_WRITE_ERRORS => e
        MCP.configuration.exception_reporter.call(
          e,
          { session_id: session_id, error: "Failed to send notification" },
        )
        if related_request_id && session[:post_request_streams]&.key?(related_request_id)
          session[:post_request_streams].delete(related_request_id)
          streams_to_close << stream
        else
          cleanup_and_collect_stream(session_id, streams_to_close)
        end
        false
      end
    else
      # Broadcast to all connected SSE sessions
      sent_count = 0
      failed_sessions = []

      @sessions.each do |sid, session|
        next unless (stream = session[:get_sse_stream])

        if session_expired?(session)
          failed_sessions << sid
          next
        end

        begin
          send_to_stream(stream, notification)
          sent_count += 1
        rescue *STREAM_WRITE_ERRORS => e
          MCP.configuration.exception_reporter.call(
            e,
            { session_id: sid, error: "Failed to send notification" },
          )
          failed_sessions << sid
        end
      end

      # Clean up failed sessions
      failed_sessions.each { |sid| cleanup_and_collect_stream(sid, streams_to_close) }

      sent_count
    end
  end

  streams_to_close.each do |stream|
    close_stream_safely(stream)
  end

  result
end

#send_request(method, params = nil, session_id: nil, related_request_id: nil) ⇒ Object

Sends a server-to-client JSON-RPC request (e.g., sampling/createMessage) and blocks until the client responds.

Uses a Queue for cross-thread synchronization. This method creates a Queue, sends the request via SSE stream, then blocks on queue.pop. When the client POSTs a response, handle_response matches it by request_id and pushes the result onto the queue, unblocking this thread.



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# File 'lib/mcp/server/transports/streamable_http_transport.rb', line 156

def send_request(method, params = nil, session_id: nil, related_request_id: nil)
  if @stateless
    raise "Stateless mode does not support server-to-client requests."
  end

  unless session_id
    raise "session_id is required for server-to-client requests."
  end

  request_id = generate_request_id
  queue = Queue.new

  request = { jsonrpc: "2.0", id: request_id, method: method }
  request[:params] = params if params

  sent = false

  @mutex.synchronize do
    unless (session = @sessions[session_id])
      raise "Session not found: #{session_id}."
    end

    @pending_responses[request_id] = { queue: queue, session_id: session_id }

    if (stream = active_stream(session, related_request_id: related_request_id))
      begin
        send_to_stream(stream, request)
        sent = true
      rescue *STREAM_WRITE_ERRORS
        if related_request_id && session[:post_request_streams]&.key?(related_request_id)
          session[:post_request_streams].delete(related_request_id)
          close_stream_safely(stream)
        else
          cleanup_session_unsafe(session_id)
        end
      end
    end
  end

  # TODO: Replace with event store + replay when resumability is implemented.
  # Resumability is a separate MCP specification feature (SSE event IDs, Last-Event-ID replay,
  # event store management) independent of sampling.
  # See: https://modelcontextprotocol.io/specification/latest/basic/transports#resumability-and-redelivery
  #
  # The TypeScript and Python SDKs buffer messages and replay on reconnect.
  # Until then, raise to prevent queue.pop from blocking indefinitely.
  unless sent
    raise "No active stream for #{method} request."
  end

  response = queue.pop

  if response.is_a?(Hash) && response.key?(:error)
    raise StandardError, "Client returned an error for #{method} request (code: #{response[:error][:code]}): #{response[:error][:message]}"
  end

  if response == :session_closed
    raise "SSE session closed while waiting for #{method} response."
  end

  response
ensure
  if request_id
    @mutex.synchronize do
      @pending_responses.delete(request_id)
    end
  end
end